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Séminaires passés

A zone of preferential heating extends tens of solar radii from Sun

Justin Kasper (Smithsonian Astrophysical Observatory, Cambridge, USA)

The extreme temperatures and non-thermal nature of the solar corona and solar wind arise from an unidentified physical mechanism that preferentially heats certain ion species relative to others. Spectroscopic indicators of unequal temperatures commence within a fraction of a solar radius above the surface of the Sun, but the outer reach of this mechanism has yet to be determined. Here we present an empirical procedure for combining interplanetary solar wind measurements and a modeled energy equation including Coulomb relaxation to solve for the typical outer boundary of this zone of preferential heating. Applied to two decades of observations by the Wind spacecraft, our results are consistent with preferential heating being active in a zone extending from the transition region in the lower corona to an outer boundary 20-40 solar radii from the Sun, producing a steady state super-mass-proportional α-to-proton temperature ratio of 5.2−5.3. Preferential ion heating continues far beyond the transition region and is important for the evolution of both the outer corona and the solar wind. The outer boundary of this zone is well below the orbits of existing spacecraft at 1 AU and even closer missions such as Helios and MESSENGER, meaning it is likely that no existing mission has directly observed preferential heating, just residual signatures. We predict that Parker Solar Probe will be the first spacecraft with a perihelia sufficiently close to the Sun to pass through the outer boundary, enter the zone of preferential heating, and directly observe the physical mechanism responsible for the high temperatures of the solar corona in action.

The Atmosphere of Pluto : Synthesis of Results from the New Horizons Mission

Darrell Strobel (Johns Hopkins University)

On 14 July 2015, NASA’s New Horizons spacecraft observed an ultraviolet solar occultation of Pluto’s atmosphere with its ALICE ultraviolet spectrograph and performed a radio occultation that sounded Pluto’s atmosphere down to the surface with radio signals transmitted simultaneously by four antennas of the NASA Deep Space Network. From the solar occultation data line-of-sight (los) optical depths that yield los column densities for 5 molecular species, and extinction coefficients for haze. The radio occultation data yield N2 number density, pressure, and temperature profiles from the surface to about 110 km of altitude at two diametric points on the planet. This talk presents a synthesis of the results from these two occultations. We find a very stable, spherically symmetry, lower atmosphere, with well-mixed portion restricted to a planetary boundary layer (surface to 12 km ; Kzz 500-4000 cm2 s-1), peak temperature of 106 K at 25 km, cold isothermal temperature 65-68 K in Pluto’s upper atmosphere, and inferred CH4 surface mixing ratio (0.28-0.35)%. The inferred enhanced Jeans escape rates are 5-7 x 1022 N2 s-1 and 5-8 x 1025 CH4 s-1 at the exobase (r 2900 km, where the Kn = 0.7).

Kostis Gontikakis (Academy of Athens, Greece)

25 Years of Adaptive Optics on Mauna Kea

Olivier Lai (Observatoire de la Côte d’Azur)

Mauna Kea is one of the premier sites worldwide in terms of atmospheric turbulence induced image quality for astronomical observation. As such it has been the site of many adaptive optics (AO) developments, starting in the early 90’s. I first came to Mauna Kea in 1994 and worked on Mauna Kea adaptive optics projects for 20 years, at Keck, CFHT and in 2013, I was jointly appointed as adaptive optics scientist between Gemini Observatory and Subaru Telescope for three years before returning to France in 2016.
In this talk, I review the developments of adaptive optics on Mauna Kea, and my personal experiences of this history. Astronomical observatories, and astrophysics itself, have undergone a profound transformation in the last twenty years towards "Big Science”. Adaptive optics instruments especially have become more expensive and are now too large to be built by observatories themselves : they are the result of multi-institute collaborations. This shift from competition to collaboration led to my Gemini-Subaru joint appointment. I will discuss some of the ideas and difficulties that emerged out of this collaborative framework, as well as the critical need to maintain small scale pathfinder experiments, illustrating the case in point with the very-wide-field GLAO prototype, `imaka.

Mardi 14 mars 2017
&agrave
11h00
(Salle de conférence du bâtiment 17)

Constraining early stellar evolution with asteroseismology

Konstanze Zwintz (Institute for Astro- and Particle Physics, University of Innsbruck)

The earliest phases in the lives of stars define their complete evolutionary paths until their deaths. Therefore, understanding the physical processes that occur in these early stages is essential. But although we have a general concept of how stars are formed and evolve, our current knowledge of early stellar evolution is limited.
Pre-main sequence stars can become vibrationally unstable during their evolution to the zero-age main sequence. As they gain their energy from gravitational contraction and have not started nuclear fusion in their cores yet, their inner structures are significantly different to those of (post-) main sequence stars.
Asteroseismology has been proven to be a successful tool to unravel details of the internal structure for different types of stars in various stages of evolution well after birth. We can now show that it has similar power for pre-main sequence objects.

Composition and distribution of clouds in exoplanets : an L/T-like transition for hot Jupiters ?

Over a large range of equilibrium temperatures clouds seem to dominate the transmission spectrum of exoplanets atmospheres but no trends allowing the classification of these objects have yet emerged. Recently, Kepler observations of the light reflected by hot Jupiters show that an inhomogeneous, asymetric and time-dependent cloud coverage is present in these planets. Interesingly, the properties of these clouds depend on the equilibrium temperature of the planet.
Using state-of-the-art three dimensional models of hot Jupiters atmospheres I will show why longitudinal and latitudinal assymetry in the cloud coverage is expected for these hot planets. The presence of such an inhomogeneous cloud coverage can bias the retrieved abundances from transmission and secondary eclipse spectra and even lead to erroneous molecular detections. The longitudinal cloud asymetry being a strong function of the condensation temperature of the cloud species, it is a telltale of the cloud composition. Observations and models converge towards a similar conclusion : an L/T-like transition is expected for hot Jupiters, with silicate clouds disappearing from the cooler planets and being replaced by manganese sulfide clouds.